In the optical measurement technical field and the optical communication technical field, optical waveguide element modules where an optical waveguide element, such as an optical modulator, is built in have been used. In most of the optical modules using a substrate having an electro-optical effect such as of lithium niobate, as shown in Patent Document 1, a bias voltage (DC voltage) for adjusting the modulation operating point relative to the high-frequency signal, which is a modulation signal, and the drive voltage is applied to the control electrodes (signal electrodes) of the optical waveguide element.
In a conventional optical modulator, as shown in FIGS. 1(a) and 1(b), a waveguide substrate 1 for forming an optical waveguide element has inside of it one modulation unit (see FIG. 1(a)) or a structure where two modulation units (see FIG. 1(b)) are connected in series, and thus, the number of external substrates 2, such as termination substrates, provided around the waveguide substrates and the number of connection terminals (T1 to T4) to be connected to an external electric circuit outside the module are small. In addition, enough space for providing the optical waveguide substrate 1 and the external substrates 2 is secured inside the housing 3, and furthermore, the layout of the connection terminals (T1 to T4), such as DC terminals, for outputting a signal to the outside can be relatively freely designed. Therefore, it is possible to easily design the layout so that the bonding wires W for connecting the waveguide substrate 1 to an external substrate 2 or an external substrate 2 to a connection terminal have an appropriate length with high reliability.
Multivalued modulation systems that correspond to a high speed, large capacity optical communication system have been used in recent years, and thus, the integration of modulation units has progressed and the number of parts used inside the housing has been increased. In addition, the locations to which input/output terminals for signals (RF connectors, DC terminals) are attached, the locations of screw holes for securing an optical modulator, and the maximum size of the optical modulators are standardized according to the international standards, and therefore, the locations to which parts are mounted inside the housing are more restricted than in the conventional compact modulators.
Therefore, the length of the bonding wires for the connection between a termination substrate and a connection terminal, such as a DC terminal for example, is longer than that of the conventional compact modulator, and in some places, it exceeds 10 mm.
In the impact/vibration test for electro-optical parts, it is stipulated that mechanical impacts are to be given in five directions under the conditions of 500 g and 1.0 ms five times per direction, and vibrations with 20 g, 20 to 2000 Hz, and intervals of 20 Hz are to be given under the conditions of four minutes per cycle and four cycles in one axial direction. When a bonding wire has a predetermined length or longer, it becomes easy to disengage or disconnect the bonding wire when vibrations or impacts are applied to the optical modulator. It is possible for the cause of this to be the lack of mechanical strength in the connection portion due to the increase of the weight of the bonding wire or an increase in the displacement of the wire due to the agreement of the length of the wire with the resonant frequency of the vibrations.
In order to prevent a bonding wire from disengaging or disconnecting, it is desirable to reduce the number of bonding wiring places or to make the bonding wires as short as possible. For example, it is possible to reduce the length of the bonding wires by placing the external substrates, such as termination substrates, close to the control electrodes or the connection terminals.
Alternatively, as shown in FIG. 2, it is possible to provide a new substrate 23 for relay between a termination substrate (21, 22) and a DC terminal (T21, T22). However, such problems arise that the number of bonding wiring places increases, the cost increases due to an increase the number of parts, and the number of work steps increases due to the mounting of additional parts.
In addition, as shown in FIG. 2, it is necessary to increase the size of the relay substrate 23 so as to fill the space between the termination substrate (21, 22) and the DC terminal (T21, T22), and thus, it also becomes difficult to make the entirety of the module compact.
Furthermore, it is desirable to use an alumina thin film substrate for the external substrates in order to obtain good high-frequency properties. In the case where lithium niobate (LN) is used for the waveguide substrate of the optical waveguide element, for example, stainless steel (SUS) is often used for the housing. In this case, the difference in the coefficient of linear thermal expansion is great between the SUS (coefficient of linear thermal expansion: 18.7×10−6/° C.) used for the housing of the modulator and the alumina (coefficient of linear thermal expansion: 7.2×10−6/° C.), and therefore, in the case where a large alumina substrate, such as the relay substrate 23 in FIG. 2), is used, the alumina substrate cracks when the optical modulator is heated at a high temperature. In order to prevent this, it is necessary to provide an intermediary material such as 50 alloy (50 Ni—Fe alloy) or 52 alloy (52 Ni—Fe alloy) between the alumina substrate and the housing, which causes a further increase in the cost.